Re: [Qemu-devel] [RFC PATCH V2 2/3] cpus: add a tcg_executing flag.

["Alex Bennée" <alex.bennee@linaro.org>]

fred.konrad@greensocs.com writes:

> From: KONRAD Frederic <fred.konrad@greensocs.com>
>
> This flag indicates if the VCPU is currently executing TCG code.
>
> Signed-off-by: KONRAD Frederic <fred.konrad@greensocs.com>
>
> Changes V1 -> V2:
>   * do both tcg_executing = 0 or 1 in cpu_exec().
> ---
>  cpu-exec.c        | 2 ++
>  include/qom/cpu.h | 3 +++
>  qom/cpu.c         | 1 +
>  3 files changed, 6 insertions(+)
>
> diff --git a/cpu-exec.c b/cpu-exec.c
> index 75694f3..2fdf89d 100644
> --- a/cpu-exec.c
> +++ b/cpu-exec.c
> @@ -371,6 +371,7 @@ int cpu_exec(CPUState *cpu)
>          cpu->halted = 0;
>      }
>  
> +    cpu->tcg_executing = 1;
>      current_cpu = cpu;
>  
>      /* As long as current_cpu is null, up to the assignment just above,
> @@ -583,5 +584,6 @@ int cpu_exec(CPUState *cpu)
>  
>      /* fail safe : never use current_cpu outside cpu_exec() */
>      current_cpu = NULL;
> +    cpu->tcg_executing = 0;
>      return ret;
>  }
> diff --git a/include/qom/cpu.h b/include/qom/cpu.h
> index efa9624..a2de536 100644
> --- a/include/qom/cpu.h
> +++ b/include/qom/cpu.h
> @@ -226,6 +226,7 @@ struct kvm_run;
>   * @stopped: Indicates the CPU has been artificially stopped.
>   * @tcg_exit_req: Set to force TCG to stop executing linked TBs for this
>   *           CPU and return to its top level loop.
> + * @tcg_executing: This TCG thread is in cpu_exec().
>   * @singlestep_enabled: Flags for single-stepping.
>   * @icount_extra: Instructions until next timer event.
>   * @icount_decr: Number of cycles left, with interrupt flag in high bit.
> @@ -322,6 +323,8 @@ struct CPUState {
>         (absolute value) offset as small as possible.  This reduces code
>         size, especially for hosts without large memory offsets.  */
>      volatile sig_atomic_t tcg_exit_req;
> +
> +    volatile int tcg_executing;

My concern is on weakly ordered backends is volatile enough for this
flag or do we need some sort of memory barrier when we update it? Does
it just introduce an inefficiency that other threads may spin a few
times waiting to find out the vCPU has halted?

If other threads are waiting for it to halt is there a mechanism that
ensures we'll never start-up again until everything is done?


>  };
>  
>  QTAILQ_HEAD(CPUTailQ, CPUState);
> diff --git a/qom/cpu.c b/qom/cpu.c
> index 4e12598..62663e5 100644
> --- a/qom/cpu.c
> +++ b/qom/cpu.c
> @@ -249,6 +249,7 @@ static void cpu_common_reset(CPUState *cpu)
>      cpu->icount_decr.u32 = 0;
>      cpu->can_do_io = 0;
>      cpu->exception_index = -1;
> +    cpu->tcg_executing = 0;
>      memset(cpu->tb_jmp_cache, 0, TB_JMP_CACHE_SIZE * sizeof(void *));
>  }

-- 
Alex Bennée